What tonearm will better mine?

I have been running a 12" Jelco SA750L with an upgraded Decca Super Gold on a Garrard 401 for a few years. I'm happy with the sound and happy with my pre, power and speakers. (Croft 25R, EICO HF35s, -good tubes, Quad 57).
I keep hearing that my tonearm is a weak link even though it sounds pretty awesome to me. What would it take to replace the Jelco and make a significant improvement? What should I expect to hear better? Would I have to drill the massive plinth? Thanks.